Among the conventionally-known centrifugal pumps are ones in which a rotation shaft rotatably projects into a volute and an impeller is mounted on the projecting rotation shaft. Such centrifugal pumps are adjustable in performance by fluid being caused to flow into a flow passage defined by the volute, as disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2012-72697 (hereinafter referred to as “the relevant patent literature”).
In the centrifugal pump disclosed in the relevant patent literature, the volute is disposed within a pump casing, and the impeller is rotatably accommodated within the volute. The volute has a surface opposed to respective one end surfaces in a rotation axis direction (i.e., respective one axial end surfaces) of vanes provided on the impeller, and a peripheral wall section extending from the outer peripheral edge of the volute's opposed surface to surround the outer periphery of the impeller. Fluid is sucked in through a suction opening formed centrally in the opposed surface, and the impeller is rotated so that, by centrifugal force, the fluid is discharged into the pump casing through a discharge opening formed in the peripheral wall section.
Although the flow passage is formed inside the volute, backward or reverse flows of the fluid are prevented by locating the axial end surfaces of the vanes and the volute's opposed surface close to each other. In order to increase a flow rate of the fluid and thereby enhance the performance of the centrifugal pump, there may be employed an approach of increasing the sectional area of the in-volute flow passage and/or increasing the diameter of the impeller. The sectional area of the in-volute flow passage can be increased by increasing a dimension of the volute in a radially outward direction so that the peripheral wall section is located more radially outward or locating the opposed surface of the volute higher in the axial direction.
If the peripheral wall section of the volute is located more radially outward and/or the opposed surface of the volute is located higher in the axial direction, however, the volute and the pump casing would interfere with each other. If the pump casing is increased in size to avoid such interference, the centrifugal pump would increase in overall size. Further, if the volute is increased in dimension in the axial direction of the rotation shaft rather than in the radially outer direction, the sectional area of the in-volute flow passage cannot be used effectively. Thus, some improvement has to be made in order to increase the flow rate while maintaining the size of the volute (i.e., without changing the size of the volute).